The present invention concerns power supplies and, more particularly, an improved resonant power supply of the flyback type, having a separate transformer winding for supplying filament heating energy to magnetron and the like load, and having circuitry for controlling power input to the load.
In the co-pending application, U.S. patent application Ser. No. 086,826, filed Oct. 22, 1979, assigned to the assignee of the present invention and incorporated here in its entirety by reference, the use of a flyback-type high-frequency, high-voltage power supply was described for energizing a self-rectifying load, such as a magnetron microwave power generator for a microwave oven and the like. In that application, a switching device, e.g. the collector-emitter circuit of a power switching transistor, is connected in series with the primary winding of a transformer to provide pulses of energy to a resonant circuit formed at the transformer secondary winding and including the electrical capacitance of the load connected across the transformer winding inductance. The load conducts only for unipolar excitation exceeding a minimum magnitude. A clamping diode and a snubber network are connected in electrical parallel across the switching device and the transformer primary winding, to protect the switching device from reverse voltage effects. This type of high-voltage power supply does not require a high-voltage rectifier; however, a separate power transformer is required to provide filament heating energy to the magnetron load. The filament transformer must be insulated, whereby the secondary winding thereof can be at a potential differing from the potential of the primary winding by several thousand volts. This insulation requirement, when considered with the requirement that the filament transformer be energized by residential power mains voltage (a 120 VAC, 60 Hz. waveform) requires that the filament transformer be of relatively great weight and volume and relatively high cost. It is therefore desirable to provide an improved power supply for energizing a magnetron, and the like loads, which does not require a separate filament transformer. It is also highly desirable to provide means for protecting the power switching device against excessive currents, while providing active power control of the load, whereby the average power consumed by the load (and the average microwave power produced by a magnetron load, specifically) is controllable.